1. Field of the Invention
This invention relates to water-based metalworking fluids and hydraulic fluids.
2. Description of the Prior Art
Thickened, water-based hydraulic fluids and metalworking fluids are known in the prior art from U.S. Pat. Nos. 4,138,346 and 4,151,099. The hydraulic fluids disclosed comprise a high molecular weight, non-associative, polyalkylene glycol thickener, a phosphate ester and a sulfur-containing compound or polyester of an oxyalkylene compound and a sulfur-containing compound alone or including a phosphate ester. A polyether-thickened, water-based hydraulic fluid or metalworking fluid is disclosed in U.S. Pat. Nos. 4,312,768 or 4,312,775. The latter patent teaches that increased thickening efficiency can be obtained utilizing a synergistic combination of an associative polyether thickener, as disclosed, with a mixture of a water-soluble amine and an ethoxylated phosphate ester. In U.S. Pat. No. 4,304,902, a copolymer of a 1,2-epoxy-n-alkane with ethylene oxide is disclosed which is solubilized in an aqueous medium by an ethoxylated surfactant to provide a substantial increase in viscosity.
In no one of these prior art references is the fact disclosed that aqueous solutions of the associative polyether thickeners are susceptible to substantially greater viscosity reduction upon heating than the non-associative polyether thickeners. Reduced temperature dependence of the viscosity of an aqueous medium utilized as a hydraulic fluid is required to more closely simulate the properties of the commonly used hydrocarbon oil-based hydraulic fluids.